package com.lb.algorithm;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * @Author: zhangjun
 * @Date: 2020/6/24 14:26
 * @Description: 给定一个仅包含数字 2-9 的字符串，返回所有它能表示的字母组合。
 *
 * 给出数字到字母的映射如下（与电话按键相同）。注意 1 不对应任何字母。
 *
 * 示例:
 *
 * 输入："23"
 * 输出：["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].
 */
public class CombinationTel {
    //定义数字和字母的对应关系
    Map<String, String> phone = new HashMap<String, String>() {{
        put("2", "abc");
        put("3", "def");
        put("4", "ghi");
        put("5", "jkl");
        put("6", "mno");
        put("7", "pqrs");
        put("8", "tuv");
        put("9", "wxyz");
    }};

    //输出集合
    List<String> output = new ArrayList<String>();

    public List<String> letterCombinations(String digits) {
        if (digits.length() != 0) {
            backtrack("",digits);
        }
        return output;
    }

    private void backtrack(String combination, String next_digits) {
        if (next_digits.length() == 0){
            output.add(combination);
        }else {
            String digit = next_digits.substring(0, 1);
            String letters = phone.get(digit);
            for (int i = 0; i < letters.length(); i++) {
                String letter = phone.get(digit).substring(i, i + 1);
                backtrack(combination + letter, next_digits.substring(1));
            }

        }
    }

    public static void main(String[] args) {
        CombinationTel tel = new CombinationTel();
        tel.letterCombinations("23").forEach(System.out::println);
    }
}
